Method for thermal treatment judgment on magneto-optical information recording medium and device for thermal treatment judgment

ABSTRACT

A laser beam from an LD  11  is irradiated onto a magnetooptic disk  33  by a predetermined power for a heat treatment. After the heat treatment, the laser beam of a power smaller than the power upon heat treatment is irradiated to the heat-treated area. Reflection light of the laser beam of the small power enters photodetectors  24  and  26  and reflection light amounts of a P wave and an S wave are detected, respectively. A differential detecting circuit  27  detects a level of a magnetooptic signal corresponding to the heat-treated area on the basis of the reflection light amounts of the P wave and the S wave. A controller  28  determines whether or not the magnetooptic signal level detected by the differential detecting circuit  28  lies within a permissible range. If it is out of the range, the power of the laser beam to execute the heat treatment is adjusted and the process of a magnetooptic disk is stopped or a message showing such a fact is displayed, or the like.

TECHNICAL FIELD

The invention relates to a heat treatment determining method and a heattreatment determining apparatus for determining whether or not a heattreatment (annealing treatment) when a magnetooptic informationrecording medium to which information is recorded by using a laser beamis manufactured has properly been executed.

BACKGROUND ART

In recent years, many magnetooptic information recording media(magnetooptic disks) have been proposed as rewritable recording media ofa high density. Among them, an attention is paid to a magnetooptic diskof a DWDD (Domain Wall Displacement Detection) system. As disclosed inthe Official Gazette of Japanese Patent No. 3332458, according to such asystem, a magnetooptic information recording medium comprising amagnetic three-layered film of at least a displacement layer, aswitching layer, and a recording layer is used and there is used such afeature that, when a signal is reproduced, a domain wall of thedisplacement layer is instantaneously moved in a region where a magneticfilm temperature is equal to or higher than a Curie temperature of theswitching layer. According to such a system, a size of magnetic domaincan be substantially enlarged and a recording density of themagnetooptic disk can be remarkably increased.

The DWDD system can be regarded as one of effective reproducing methodsin terms of a point that a very large signal can be reproduced even froma small recording magnetic domain corresponding to a period which isequal to or less than optical limit resolution of reproduction light andthe high density can be realized without changing a wavelength of light,a numerical aperture (NA) of an objective lens, or the like.

The magnetooptic disk of the general DWDD system has a construction asshown in FIG. 10. A magnetooptic information recording medium 140 shownin FIG. 10 is constructed in such a manner that a first dielectric layer142, a displacement layer 143, a switching layer 144, a recording layer145, a second dielectric layer 146, and a protecting layer 147 arelaminated on a substrate 141 in this order. The substrate 141 is atransparent substrate made of, for example, glass, polycarbonate,polyolefin, or the like.

The first dielectric layer 142 is made of, for example, SiN, AlN, or thelike and has a thickness of about 30 nm. The displacement layer 143 ismade of a perpendicular magnetic film in which a domain wall coerciveforce is relatively smaller and a domain wall displacement speed isrelatively larger than those of the recording layer 145 and is, forexample, a GdFeCo layer having a thickness of 30 to 60 nm.

The switching layer 144 has a Curie temperature lower than those of thedisplacement layer 143 and the recording layer 145 and is, for example,a GdFeCoAl layer having a thickness of 10 to 15 nm.

The recording layer 145 is, for example, a TbFeCo layer having athickness of about 50 nm. The second dielectric layer 146 is made of,for example, SiN, AlN, or the like and has a thickness of about 30 nm.The protecting layer 147 is, for example, a UV (ultraviolet) cured resinhaving a thickness of 5 to 10 μm. Those layers are laminated on thesubstrate 141 on which guide grooves (tracks) have previously beenformed.

The guide grooves of the substrate 141 are formed as shown in, forexample, FIG. 11. The magnetooptic information is recorded in a widewidth portion in FIG. 11, that is, on a land 151 and a groove 152 and amagnetic layer laminated in a wall surface portion 153 becomes a targetof the heat treatment (annealing treatment). By the heat treatment, theportion of the magnetic layer serving as a treatment target isnon-magnetized or becomes an in-plane magnetic film.

The land denotes a portion on a remote side from a surface (for example,under surface of FIG. 11) where a laser beam for recording/reproductionis inputted. A portion on a near side from such a surface is called agroove. The laser beam for the heat treatment is irradiated from thesurface (for example, top surface of FIG. 11) opposite to the side ofthe laser beam for recording/reproduction.

Since an area between the tracks is heat-treated, in the case ofrecording onto both of the land and the groove, the wall surface portion153 is heat-treated. However, in the case of recording data onto one ofthe land and the groove, the other is heat-treated.

The reproduction of a signal according to the DWDD system will now bedescribed with reference to FIGS. 12A to 12E. FIG. 12A shows an exampleof a cross sectional view of the magnetooptic information recordingmedium which is used for reproduction of the DWDD system. This medium isillustrated upside down from the magnetooptic information recordingmedium 140 shown in FIG. 10. In a manner similar to the magnetoopticinformation recording medium 140 of FIG. 10, a magnetic layer comprisinga displacement layer 160, a switching layer 161, a recording layer 162is formed. In the state where a reproduction laser beam 163 is notirradiated, in each layer, a switched coupling force acts and an atomicspin in each of the displacement layer 160 and the switching layer 161is oriented in the same direction as that of an atomic spin 164 in therecording layer 162. A domain wall 165 is formed in a boundary portionof the adjacent atomic spins (the directions of the atomic spins areopposite).

When the reproduction laser beam 163 is irradiated to the magnetoopticinformation recording medium, for example, distribution of a temperatureT of the magnetic layer as shown in FIG. 12B is obtained. Thereproduction laser beam 163 is irradiated from a substrate side as shownin FIG. 12A. Ts denotes a Curie temperature of the switching layer 161.In association with such temperature distribution, distribution of adomain wall energy density σ is formed as shown in FIG. 12C. Generally,since the domain wall energy density decreases in accordance with anincrease in temperature of the magnetic layer, distribution in which thedensity becomes lowest at the position of the highest temperature shownin FIG. 12B is obtained. Thus, a domain wall driving force F(x) to movethe domain wall 165 in the direction of the low domain wall energydensity, that is, in the direction of the high temperature of themagnetic layer. The distribution of the domain wall driving force F(x)is shown in FIG. 12D.

When there is a gradient (change) of the domain wall energy density asmentioned above, the domain wall driving force F(x) shown by thefollowing equation (1) acts on the domain wall of each layer.F(x)=−∂σ/∂x   (1)

The domain wall driving force F(x) acts so as to move the domain wall165 in the direction of the low domain wall energy density. That is, atthe position where the temperature of the magnetic layer is lower thanthe Curie temperature Ts of the switching layer, since the layers aremutually switched-coupled even if the domain wall driving force F(x) dueto such a temperature gradient acts, the movement of the domain walldoes not occur because it is blocked by a large domain wall coerciveforce of the recording layer. However, at the position where thetemperature of the magnetic layer is higher than the Curie temperatureTs of the switching layer, since the switched-coupling between thedisplacement layer 160 and the recording layer 162 is cut, the domainwall of the displacement layer 160 whose domain wall coercive force issmall can be moved by the domain wall driving force F(x) according tothe temperature gradient. Therefore, when the reproduction laser beam163 is irradiated upon scanning of the magnetooptic informationrecording medium, at the moment when the domain wall exceeds theposition of the Curie temperature Ts and enters the coupling switchingarea, the domain wall of the displacement layer 160 moves toward thehigh temperature side (direction shown by an arrow 166 in FIG. 12A).

By the principle as mentioned above, the domain walls formed on themagnetooptic information recording medium at intervals corresponding tothe recording signal are moved every scan which is executed by the laserbeam. Thus, a size of magnetic domain effectively recorded is enlargedupon reproduction, a reproduction carrier signal can be increased, andthe reproduction exceeding the optical limit can be performed. Awaveform shown in FIG. 12E relates to an example of a reproductionwaveform which is obtained from the magnetic layer in FIG. 12A. In thisinstance, the signal at the low level is obtained when the atomic spinsin the recording layer 162 are oriented downwardly.

The equation (1) showing the domain wall driving force F(x) in thereproduction by the DWDD system is inherently derived from the followingequation (2).F(x)=2M(x)·Hd(x)+2M(x)·Ha−σ(x)/x−∂σ/∂x   (2)where,

-   -   M(x): magnetization of the displacement layer 160    -   Hd(x): demagnetizing field    -   Ha: external magnetic field such as a leakage flux or the like        from the recording layer 162    -   σ(x): domain wall energy per unit area

For example, as disclosed in “Journal of Magnetic Society of Japan”,Vol. 22, Supplement No. S2, 1998, pp. 47-50, by extremely reducing themagnetization of the displacement layer 160, the first term(2M(x)·Hd(x)) and the second term (2M(x)·Ha) of the right side of theequation (2) can be ignored. Further, if the apparatus is constructed sothat no closed magnetic domains are formed by, for example,non-magnetizing both sides of the recording track (guide groove) orconverting them into in-plane magnetic films by heat-treating them, thethird term (−σ(x)/x) of the right side of the equation (2) can beignored. Therefore, by remarkably reducing the magnetization of thedisplacement layer 160 and by non-magnetizing both sides of therecording track or converting them into in-plane magnetic films byheat-treating them, the right side of the equation (2) is constructedonly by the fourth term and is equal to the equation (1), so that thereproduction by the DWDD system can be executed.

Consequently, the process for non-magnetizing both sides of therecording track or converting them into in-plane magnetic films byheat-treating them is a very important process in order to realize theabove system. By the heat treatment, magnetic anisotropy of the heatingportion deteriorates and the magnetic coupling is weakened. In the heattreatment (also referred to as initialization or annealing treatment),since a track density can be raised by executing the heat treatment to anarrow area between the tracks, a spot smaller than the spot which isused for recording/reproduction of the magnetooptic informationrecording medium is often used. That is, an apparatus for executing theheat treatment is often prepared separately from the apparatus forexecuting the recording/reproduction.

However, in the foregoing magnetooptic information recording medium,even if the heat treatment is executed, a change in magnetism merelyoccurs in the treatment target portion and a method of determining orinspecting whether or not the heat treatment has properly been executeddoes not exist. On the other hand, if a width of heat treatment (heattreatment power) is too small, although recording performance of thetrack is improved, a proper recording power margin cannot be assured. Ifthe width is too large, the recording performance of the trackdeteriorates.

It is, therefore, an object of the invention to provide a heat treatmentdetermining method and a heat treatment determining apparatus of amagnetooptic information recording medium, in which whether or not aheat treatment has properly been executed, in other words, whether ornot a proper width has been heat-treated (by a proper power) can beeasily determined.

DISCLOSURE OF INVENTION

According to the invention, there is provided a heat treatmentdetermining method comprising the steps of: executing a heat treatmentof a magnetic layer by irradiating a laser beam of a first power to anarea between tracks of a magnetooptic information recording mediumobtained by laminating the magnetic layer onto a substrate on which thetracks have previously been formed, in which the magnetic layer isconstructed by a recording layer to hold recording magnetic domainsaccording to recording information, a displacement layer made of aperpendicular magnetic film whose domain wall coercive force is smallerand whose domain wall displacement speed is higher than those of therecording layer, and a switching layer which is arranged between therecording layer and the displacement layer and whose Curie temperatureis lower than those of the recording layer and the displacement layer;irradiating a laser beam of a second power smaller than the first powerto the heat-treated area; detecting a level of a magnetooptic signalfrom reflection light of the laser beam of the second power; anddetermining whether the heat treatment is proper or improper on thebasis of the detected magnetooptic signal.

According to the invention, there is provided a heat treatmentdetermining apparatus comprising: heat treatment means for executing aheat treatment of a magnetic layer by irradiating a laser beam of afirst power to an area between tracks of a magnetooptic informationrecording medium obtained by laminating the magnetic layer onto asubstrate on which the tracks have previously been formed, in which themagnetic layer is constructed by a recording layer to hold recordingmagnetic domains according to recording information, a displacementlayer made of a perpendicular magnetic film whose domain wall coerciveforce is smaller and whose domain wall displacement speed is higher thanthose of the recording layer, and a switching layer which is arrangedbetween the recording layer and the displacement layer and whose Curietemperature is lower than those of the recording layer and thedisplacement layer; irradiating means for irradiating a laser beam of asecond power smaller than the first power to the heat-treated area;detecting means for detecting a level of a magnetooptic signal fromreflection light of the laser beam of the second power; and determiningmeans for determining whether the heat treatment is proper or improperon the basis of the detected magnetooptic signal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a construction of a heat treatmentdetermining apparatus according to the first embodiment of theinvention.

FIG. 2 is a graph showing an example of a relation between a heattreatment power and a magnetooptic signal.

FIG. 3 is a graph showing an example of a relation between the heattreatment power and a recording power margin.

FIG. 4 is a block diagram showing a construction of a heat treatmentdetermining apparatus according to the second embodiment of theinvention.

FIG. 5 is a schematic diagram showing a structure of a groove substrate.

FIG. 6 is a schematic diagram showing a structure of a sampling servosubstrate.

FIG. 7 is a flowchart showing an example of the operation of acontroller in the heat treatment determining apparatus of the invention.

FIGS. 8A and 8B are schematic diagrams showing layout examples of spotsof laser beams which are irradiated onto a magnetooptic disk.

FIGS. 9A and 9B are schematic diagrams showing layout examples of thespots of the laser beams which are irradiated onto the magnetoopticdisk.

FIG. 10 is a schematic diagram showing a cross section of a magnetoopticdisk of the DWDD system.

FIG. 11 is a schematic diagram showing a structure of the magnetoopticdisk shown in FIG. 10.

FIGS. 12A to 12E are schematic diagrams for use in explanation of aprinciple of the DWDD system.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the invention, in order to determine whether or not a heattreatment in a magnetooptic disk has properly been executed, a heattreatment target portion in which a magnetization change is caused bythe heat treatment is read by a laser pickup, thereby detecting amagnetooptic signal (MO signal) in this portion. At this time, it isdesirable that a proper modulation signal has been recorded in theportion to be heat-treated or such a portion has been magnetized in onedirection.

First, the first embodiment of a heat treatment determining apparatusfor determining whether or not the heat treatment (annealing treatment)of a magnetic layer between tracks of the magnetooptic informationrecording medium constructed as mentioned above has properly beenexecuted will be described with reference to FIG. 1. The heat treatmentdetermining apparatus of the embodiment executes the conventional heattreatment and determines whether or not the heat treatment has properlybeen executed by detecting a magnetooptic signal of the heat-treatedportion. A series of component elements including an LD 11, a collimator12, a servo circuit 17, and the like, which will be explainedhereinafter, corresponds to heat treatment means for executing theconventional heat treatment and irradiating means for irradiating alaser beam to detect a level of the magnetooptic signal. Photodetectors24 and 26 and a differential detecting circuit 27 (and a power monitor40), which will be explained hereinafter, correspond to detecting meansfor detecting the level of the magnetooptic signal. A controller 28corresponds to determining means for determining whether the heattreatment is proper or improper on the basis of the detected level ofthe magnetooptic signal. The controller 28 controls the operations ofother component elements of a heat treatment determining apparatus 10.

The heat treatment determining apparatus 10 of the embodiment uses thelaser diode (LD) 11 for oscillating a laser whose wavelength is equal toabout 400 nm as a light source and uses an objective lens 16 whosenumerical aperture is equal to 0.85. The above component elements arenothing but an embodiment of the invention and other various lightsources, lenses, and the like can be used in the heat treatmentdetermining apparatus 10.

A laser beam (light beam) which is oscillated from the LD 11 isconverted into parallel light by the collimator (collimator lens) 12,thereafter, passes through a shaping prism 13, and is separated intothree laser beams, that is, one 0-order diffraction light and two kindsof first-order diffraction light by a diffraction grating 14. Thoselaser beams are separated by a beam splitter 15 into, for example, lightwhich is directed toward a power monitor 31 including a photodiode andlight which is directed toward a magnetooptic disk 33. A detectionresult of the power monitor 31 is transmitted to an APC (Auto PowerControl) 30 and the APC 30 controls a laser driver 29 so as to controlan output of the LD 11.

The three laser beams directing toward the magnetooptic disk 33 areconverged by the objective lens 16 and irradiated onto the magnetoopticdisk 33. For example, a 0-order diffraction laser beam 35 is a laserbeam for the heat treatment or the heat treatment determination.First-order diffraction laser beams 34 and 36 are laser beams fortracking. The three laser beams 34 to 36 are reflected by themagnetooptic disk 33 and enter a photodetector 20 through the beamsplitter 15. The photodetector 20 has a photodetector for a servo. Thereflection light of each of the laser beams 34 and 36 is detected by thephotodetector for the servo in order to perform a tracking servo. Thereflection light of the laser beam 35 is detected by a detector for aservo in order to perform a focusing servo. Although not shown, anoptical head including the objective lens 16 can be moved in the radialdirection of the magnetooptic disk 33 by an optical head transportingmechanism. Further, the controller 28 transmits a control signal to aspindle driver 32 and controls the rotation of the magnetooptic disk 33.

The laser beam from the LD 11 is actuator-controlled by the focusingservo and the tracking servo so that a focal point is in-focused ontothe magnetooptic disk 33 and a predetermined position is traced. Thefocusing servo control and the tracking servo control are realized bythe servo circuit 17.

The reflection light of the laser beam 35 is separated by a beamsplitter 18 into light which is directed toward a λ/2 plate 21 and,further, separated into light which is directed toward the photodetector24 and light which is directed toward the photodetector 26,respectively. In the embodiment, for example, a P wave is inputted tothe photodetector 24 and an S wave is inputted to the photodetector 26.There is such a nature that when the laser beam 35 is reflected by themagnetized surface of the magnetooptic disk 33, a polarization plane ofthe reflection light is rotated in accordance with the magnetic field.Such a nature is called a Kerr effect.

As will be explained hereinafter, when the heat treatment power isincreased, the magnetooptic signal which is obtained from the reflectionlight from the portion where the heat treatment has been performedchanges gradually. According to the invention, an attention is paid tosuch a nature, the magnetooptic signal in the portion where the heattreatment has been performed is detected from the P wave and the S wave,and a magnitude of the heat treatment power is determined, that is,whether the heat treatment is proper or improper is determined on thebasis of the detection result.

Although the embodiment is made on the assumption that when amagnetooptic film of the magnetooptic disk is formed, the magnetizingdirection is deviated to a predetermined direction, it is also possibleto construct in such a manner that before the heat treatment, forexample, the heat treatment target portion is preliminarily magnetizedin one direction by a laser beam 38 and a magnetic head 39 shown inFIG. 1. By this construction, the above determination can be easilymade. In place of the magnetization, if a modulation signal ispreliminarily recorded in the heat treatment target portion and anamplitude of the magnetooptic signal is detected from the reflectionlight, the signal amount is increased to twice as large as that in thecase where the magnetization is executed. Therefore, the abovedetermination can be made much easier.

The uniform magnetization of the magnetooptic disk or the recording ofthe modulation signal may be simultaneously executed. That is, while thelaser beam which is used for the heat treatment is tracing, a uniformmagnetic field or a modulation magnetic field is applied. For example,an objective lens with a magnetic coil is used as means for generating amagnetic field for recording. In the invention, for example, since ahigh frequency is unnecessary as a modulation signal, a magnetic head 37arranged on the substrate side may be used. Further, in the case ofexecuting the uniform magnetization, a permanent magnet may be used.

Since the uniform magnetization or the recording of the modulationsignal as mentioned above cannot be executed after the heat treatment isexecuted, the determination about whether the heat treatment is properor improper can be made by using such a principle. That is, after theheat treatment was executed, the uniform magnetization or the recordingof the modulation signal is executed by the laser beam 38 and themagnetic head 39 by a smaller power adapted to perform the heattreatment to the portion where the heat treatment was executed. Afterthat, a reproduction signal, that is, the magnetooptic signal of thisportion is detected by the foregoing photodetectors 24 and 26 anddifferential detecting circuit 27. If the heat treatment has properlybeen executed, since the recording to this portion cannot be executed,the magnetooptic signal corresponding to such a state is obtained. Onthe other hand, if the heat treatment is not properly executed due tosome reasons, the magnetooptic signal at the level corresponding to sucha state is obtained.

The reflection light amounts detected by the photodetectors 24 and 26are inputted to the differential detecting circuit 27. In thedifferential detecting circuit 27, if only the P wave or the S wave isused, an amplitude difference is small. Therefore, the amplitude isdoubled by overlaying those two signals, thereby improving detectingprecision of the magnetooptic signal. Since the same noise is generatedin both of the P wave and the S wave, the noises can be also eliminatedby using those two signals.

The magnetooptic signal detected by the differential detecting circuit27 is sent to the power monitor 40. For example, the power monitor 40converts the detection result into the digital signal and transfers itto the controller 28. As necessary, the power monitor 40 displays thevalues of the digital magnetooptic signal onto a display or the like.

From the received detection result, the controller 28 determines whetheror not it is the proper magnetooptic signal (amplitude). The detectionresult can be used for feedback control of the heat treatment power sothat the constant proper heat treatment is continuously executed. Thecontrol of the heat treatment power is made by a method whereby, forexample, a predetermined command is transmitted from the controller 28to the laser driver 29 and the output of the LD 11 is controlled. Whenthe received magnetooptic signal does not have a proper value, thecontroller 28 can display an alarm indicative of such a fact or stop theprocess of the magnetooptic disk.

A push-pull method is used for the tracking servo mechanism in theembodiment. That is, control is made in such a manner that the twotracking laser beams (34, 36) are irradiated onto therecording/reproducing tracks (land 151, groove 152) adjacent to the wallsurface portion 153 in FIG. 11 serving as a target of the heat treatmentor the heat treatment determination, respectively, and by calculatingthe reflection light amounts, the laser beam 35 is certainly irradiatedonto the wall surface portion 153. An area of the spot of the laser beam35 on the magnetooptic disk 33 is generally smaller than that of thespot of each of the laser beams (34, 36).

As mentioned above, since the laser beam for determining whether or notthe heat treatment has properly been executed also traces the same wallsurface portion 153 as that for the heat treatment, an apparatus similarto the apparatus for executing only the heat treatment of themagnetooptic disk can be used as tracking means. For example, the methodused in the optical disk heat treatment apparatus disclosed inJP-A-2002-319201 can be applied.

In the above optical disk heat treatment apparatus, the laser beam isseparated by a diffraction grating or the like into three kinds of laserbeams such as laser beam for the heat treatment and two laser beams fortracking and the tracking for the heat treatment is realized by thesampling servo system using wobble pits. In the embodiment, it isapplied to the land/groove substrate so as to be realized by thepush-pull system. That is, as mentioned above, one of the tracking laserbeams traces the land 151 and the other traces the groove 152, therebyallowing the laser beam for the heat treatment determination to trace aboundary portion (wall surface portion 153) between the land 151 and thegroove 152.

The laser beam is first irradiated onto the wall surface portion 153 ofthe magnetooptic disk 33 as mentioned above, so that the magnetic layerof this portion is non-magnetized or converted into the in-planemagnetic film (that is, heat-treated). A width to be heat-treatedincreases in association with an increase in LD power.

In the embodiment, after the irradiation of the laser beam for the heattreatment is performed to at least a predetermined range of the wallsurface portion 153 of the magnetooptic disk 33, the laser beam of apower smaller than that of the laser beam for the heat treatment isirradiated again to the wall surface portion 153 in such a range, andthe magnetooptic signal is detected from the reflection light, therebydetermining whether or not the heat treatment has properly beenexecuted. For example, the innermost rim track of the magnetooptic diskis set as a test zone. The heat treatment is executed in the test zone,after that, the magnetooptic signal is detected, and whether or not theheat treatment has properly been executed is determined. If it isdetermined that the heat treatment has properly been executed, the heattreatment is executed with respect to all of the residual tracks by theheat treatment power used for the heat treatment.

A procedure to specify the proper magnetooptic signal will now bedescribed. FIG. 2 is a graph showing an example of a relation betweenthe heat treatment power and the magnetooptic signal. A relative speedof the magnetooptic disk to the laser beam at the time of the heattreatment is equal to, for example, 4 m/sec. An axis of ordinate(magnetooptic signal) of the graph of FIG. 2 indicates the level of themagnetooptic signal (MO signal) obtained from the P wave and the S waveof the reflection light when the heat-treated wall surface portion 153is traced by the low laser power of 0.5 mW after completion of the heattreatment. A numerical value of the axis of ordinate of the graph isshown as a relative value in which a predetermined value is set to areference. Quality of the magnetooptic signal in the case where therecording/reproduction of the magnetooptic disk has been executed isdetermined by the width of beam spot in the heat treatment.

As will be understood from FIG. 2, when the heat treatment power isequal to about 2 to 3 mW, there is no large change in the level of themagnetooptic signal. However, when the heat treatment power is equal toabout 3 to 6 mW, as the heat treatment power increases, the level of themagnetooptic signal decreases. It is desirable that the magnetoopticdisk has been DC magnetized before the heat treatment, during the heattreatment, or after the heat treatment or the modulation signal has beenrecorded. Thus, the level of the magnetooptic signal after the heattreatment changes from that before the heat treatment and a change ratevaries depending on the heat treatment power.

FIG. 3 is a graph showing an example of a relation between the heattreatment power and a recording power margin and a relation between theheat treatment power and a bit error rate. A wavelength of the LD of themagnetooptic signal recording/reproducing apparatus used for therecording/reproduction of the magnetooptic disk is equal to 660 nm. Anumerical aperture of the objective lens is equal to 0.6. In the aboveapparatus, the laser beam is irradiated from the substrate side. Agroove pitch of the magnetooptic disk used in this example is equal to1.08 μm and a track pitch is equal to 0.54 μm.

The recording power margin denotes a margin of the recording power inwhich the bit error rate is equal to or less than a predetermined value.With respect to overwriting characteristics and cross-writingcharacteristics, the bit error rate changes depending on the recordingpower. However, the recording power margin is obtained from an upperlimit value, a lower limit value, and an intermediate value of therecording power in which the bit error rate is equal to or less than thepredetermined value, for example, 10⁻⁴. For example, assuming that theupper limit value=0.9 and the lower limit value=1.1, the intermediatevalue=1.0 and a range which satisfies the above bit error rate is arange of 10% in the direction of the upper limit or the lower limit fromthe intermediate value. In this case, the recording power margin isequal to +/−10%. The recording power is a power forrecording/reproducing the magnetooptic information onto/from themagnetooptic disk and differs from the heat treatment power.

Such a recording power margin changes depending on the heat treatmentpower which is used upon execution of the heat treatment. In FIG. 3, acurve shown by a broken line indicates the relation between the heattreatment power and the recording power margin. An axis of ordinate ofthe left side denotes a scale of the recording power margin and its unitis set to (+/2)%. In this example, the recording power margin occurs ina range where the heat treatment power is equal to about 3.5 to about 6mW and the recording power margin becomes a peak (about +/−18%) when theheat treatment power is equal to about 5 mW.

On the other hand, a curve shown by a solid line indicates the relationbetween the heat treatment power and the bit error rate. In moredetails, it shows a bottom (minimum value) of the bit error rate at theheat treatment power. A scale of the bit error rate is set on an axis ofordinate on the right side.

In the quality management upon manufacturing of the magnetooptic disk,such a recording power margin is often used as a reference. It is nowassumed that a value of +/−16% or more is required as a recording powermargin. Thus, it will be understood that it is necessary to set the heattreatment power to a value within a range from about 4.5 to 5.2 mW.Subsequently, referring to FIG. 2 again, it will be understood that thelevel of the magnetooptic signal corresponding to the range (that is,from 4.5 to 5.2 mW) of the heat treatment power is equal to about 0.3 to0.6.

In this case, therefore, in the heat treatment determination of themagnetooptic disk, the disk in which the magnetooptic signal accordingto the reflection light is at the level of about 0.3 to 0.6 passes aninspection or it is necessary to control the heat treatment so as toobtain such reflection light. If the heat treatment is executed whilemonitoring the reflection light as mentioned above, a mistake of theheat treatment step can be prevented. Since the correlations shown inFIGS. 2 and 3 are influenced by a delicate structure or the like of themagnetooptic disk, those characteristics can also change on a lot unitbasis (for example, 1000 magnetooptic disks) or a unit less than 1000.

A heat treatment determining apparatus according to the secondembodiment of the invention will now be described with reference to FIG.4. In the heat treatment determining apparatus of the first embodimentmentioned above, the laser beam from the LD 11 is separated into thethree laser beams by using the diffraction grating 14. One laser beam isused for the heat treatment or the detection of the reflection lightamount and the two residual laser beams are used for tracking. However,it is also possible to construct in such a manner that two LDs areprepared, one of them is used for the heat treatment or the detection ofthe magnetooptic signal and the other is used for tracking. A series ofcomponent elements including an LD 61, a collimator 62, a servo circuit67, and the like, which will be explained hereinafter, corresponds tothe heat treatment means for executing the conventional heat treatmentand the irradiating means for irradiating the laser beam to detect thelevel of the magnetooptic signal. Photodetectors 74 and 76 and adifferential detecting circuit 77 (and a power monitor 91), which willbe explained hereinafter, correspond to the detecting means fordetecting the level of the magnetooptic signal. A controller 78corresponds to the determining means for determining whether the heattreatment is proper or improper on the basis of the level of themagnetooptic signal detected as mentioned above. The controller 78controls the operations of other component elements of a heat treatmentdetermining apparatus 60.

In the heat treatment determining apparatus 60 in FIG. 4, the LD 61 is alaser beam source to perform the irradiation for the heat treatment andthe detection of the magnetooptic signal and an LD 80 is a laser beamsource for tracking. The laser beam from the LD 61 is irradiated (laserbeam 87) onto a magnetooptic disk 86 through the collimator 62, ashaping prism 63, beam splitters 64 and 65, and an objective lens 66.Reflection light of the laser beam 87 irradiated in this manner isdetected by the photodetectors 74 and 76. The photodetectors 74 and 76detect reflection light amounts of the P wave and the S wave,respectively, in a manner similar to the photodetectors 24 and 26described with respect to FIG. 1.

Detection results from the photodetectors 24 and 26 are sent to thedifferential detecting circuit 77 and the level of the magnetoopticsignal is detected there. A detection result is transferred to the powermonitor 91, by which digital conversion or the like of the detectedlevel of the magnetooptic signal is executed and the digital signal istransferred to the controller 78. The controller 78 determines whetherthe heat treatment is proper or improper on the basis of the supplieddetection result and controls laser drivers 79 and 82, a spindle driver85, or the like in accordance with the detection result, therebyallowing the proper heat treatment to be executed.

The laser beam from the LD 80 is irradiated (laser beam 87) onto themagnetooptic disk 86 through a collimator 81, the beam splitters 64 and65, and the objective lens 66. Reflection light of the laser beam 87irradiated in this manner is detected by a photodetector 68 through thebeam splitter 65 and a beam splitter 70. Tracking control is made by theservo circuit 67 on the basis of its detection result. The beamsplitters 64, 65, and 72 are polarization beam splitters.

In the heat treatment determining apparatus of the embodiment, since itis presumed that a land/groove substrate having the lands and grooves isused as a magnetooptic disk 86, the laser beam for the heat treatment(and for the detection of the magnetooptic signal) and the laser beamfor the tracking have separately been prepared as mentioned above.However, in what is called a groove substrate as shown in FIG. 5 or asampling servo substrate formed with guide grooves as shown in FIG. 6,there is no need to additionally prepare the laser beam for the tackingin order to heat-treat the land portion. The groove substrate of FIG. 5includes grooves 93 and lands 94 shown by hatched regions and theportions of the lands 94 become targets for the heat treatment and theheat treatment determination. The sampling servo substrate of FIG. 6 isconstructed by grooves 95, lands 96, and pits 97 and 98. The lands 96become targets for the heat treatment and the heat treatmentdetermination.

In the sampling servo substrate of FIG. 6, the tracking is executed tothe portions of the lands 96 at the time of the heat treatment and theheat treatment determination. The pits 98 are traced by the laser beamof the large spot upon recording/reproduction, thereby allowing atracking error signal to be generated.

A heat treatment determining apparatus according to the third embodimentof the invention will now be described. The heat treatment determiningapparatus of the first embodiment uses a construction in which the powerof the laser beam used for the heat treatment is reduced and theheat-treated portion is traced again by the reduced power, and an amountof the reflection light is detected. However, the detecting process canbe executed by another method. For this purpose, in the heat treatmentdetermining apparatus of the third embodiment, a second optical pickupdifferent from the first optical pickup for the heat treatment ismounted. The heat-treated portion is traced by the second optical pickupby the power lower than the LD power which is used for the heattreatment, thereby determining whether or not the heat treatment hasproperly been executed. By such a construction, the first optical pickupand the second optical pickup can be made operative in parallel and thetime of the heat treatment step including the heat treatmentdetermination can be shortened.

In a heat treatment determining apparatus according to the fourthembodiment of the invention, two LDs are used for the magnetooptic diskas shown in FIG. 5 or 6. That is, the two LDs are arranged in such amanner that a laser beam from one of the LDs is used in common for theheat treatment and for the tracking and traces the land and, after theheat treatment, a laser beam from the other LD traces the heat-treatedportion (land) in order to detect the reflection light amount. Accordingto this construction, whether or not the heat treatment has properlybeen executed can be determined by the subsequent laser beam.

A heat treatment determining apparatus according to the fifth embodimentof the invention is made to improve the heat treatment determiningapparatus of the first embodiment. In the first embodiment, the laserbeam is separated into the three laser beams by the diffraction grating14, the 0-order diffraction light is used for the heat treatment, andthe first-order diffraction light is used for the tracking. However, inthe fifth embodiment, the 0-order diffraction light is used for the heattreatment and for the tracking. The preceding first-order diffractionlight is used to trace the heat treatment portion which is traced by the0-order diffraction light and the subsequent first-order diffractionlight is used for detection of the reflection light amount. To preventthe heat treatment from being executed by the first-order diffractionlight, it is desirable that the first-order diffraction light issufficiently small as a spectrum ratio. Because of a similar reason, itis desirable that a spot size of the first-order diffraction light islarger than that of the 0-order diffraction light.

An example of the operation of the controller 28 shown in FIG. 1 or thecontroller 78 shown in FIG. 4 will now be described with reference to aflowchart of FIG. 7. A case where the controller controls the processfor the heat treatment and the process for determining whether or notthe heat treatment has properly been executed is considered here. Theheat treatment process and the determining process are executed inparallel. The portion which was heat-treated by the heat treatmentprocess is determined by the determining process after a little whileafter completion of the heat treatment. The flowchart of FIG. 7 showsthe determining process.

In the heat treatment process, the laser beam is irradiated, forexample, onto the wall surface portion 153 of the magnetooptic disk bythe heat treatment power which has been preset. If the heat treatmentwas executed by the improper heat treatment power due to some causes,the determining process detects such a fact and executes a predeterminedprocess.

The determining process of FIG. 7 will be described hereinbelow. First,in step S1, the controller (28, 78) controls in such a manner that thelaser power of the power lower than the power used for the heattreatment is irradiated to the heat-treated portion and the level of themagnetooptic signal is detected from the reflection light amount. Thereflection light amount is obtained through the photodetector (24, 26,74, 76) and provided, for example, as a predetermined current value tothe differential detecting circuit (27, 77). The differential detectingcircuit (27, 77) detects the level of the magnetooptic signal from theprovided reflection light amount and supplies it to the controller (28,78) through the power monitor (40, 91). In step S2, whether or not thedetected level of the magnetooptic signal lies within a permissiblerange. The permissible range of the magnetooptic signal level isdetermined, for example, as mentioned in FIGS. 2 and 3. That is, theheat treatment power at which the recording power margin of apredetermined value or more is obtained is derived from FIG. 3. Therange of the magnetooptic signal level corresponding to the heattreatment power obtained in this manner is derived from FIG. 2 and setto the range of the magnetooptic signal level mentioned above. In theexample of FIG. 2, it is a range from 0.3 to 0.6. This range of themagnetooptic signal level is set, for example, every manufacturing lotof the magnetooptic disk, stored into a memory or the like in thecontroller (28, 78), and referred to at the time of the abovedetermination.

If it is determined in step S2 that the magnetooptic signal level lieswithin the permissible range, whether or not the determination targetsstill exist is determined in step S3. If there are no determinationtargets, the process is finished. If the determination targets remain,step S4 follows and whether or not the detected level of themagnetooptic signal lies within a predetermined range narrower than thepermissible range is determined. If it is decided in step S4 that thedetected magnetooptic signal level is out of the predetermined range,step S5 follows. The setting of the heat treatment power is changed sothat the optimum recording power margin can be obtained and the changedheat treatment power is transmitted to the heat treatment process whichoperates in parallel. If it is sufficient merely to determine whether ornot the magnetooptic signal level lies within the permissible range,steps S4 and S5 can be also omitted.

If it is determined in step S4 that the magnetooptic signal level lieswithin the predetermined range, step S6 follows and the position of themagnetooptic disk is controlled so as to make a determination of thenext heat treatment portion. The processing routine is returned to stepS1. In the case where the setting of the heat treatment power is changedin step S5, the processing routine also advances to step S6 and, afterthat, is returned to step S1.

If it is determined in step S2 that the detected magnetooptic signallevel is out of the permissible range, step S7 follows and a messageshowing such a fact is displayed on a display apparatus or the like. Theheat treatment of the magnetooptic disk which has precedently beenexecuted is stopped in step S8 and the determining process is alsofinished. Such a procedure is taken because it is decided that themagnetooptic disk which is being processed does not satisfy the requiredquality. With respect to the subsequent heat treatment of themagnetooptic disk, some improvement such as resetting of the heattreatment power or the like is requested. In this example, when themagnetooptic signal level is out of the permissible range, the heattreatment process and the determining process are stopped as mentionedabove. However, another countermeasure method can be also used.

The operation of the controller as mentioned above can be realized bycontrol by a microcomputer, control by a CPU based on commands of aprogram loaded in a memory, or the like.

A positional relation of the spots of the laser beams which areirradiated onto the magnetooptic disk will now be described withreference to FIGS. 8A, 8B, 9A, and 9B. FIG. 8A shows an example of alayout of the spots in the case of using the heat treatment determiningapparatus according to the first embodiment. According to theconstruction of the magnetooptic disk shown in the diagram, it is aland/groove substrate similar to that of FIG. 11 and is constructed bythe lands 151, grooves 152, and wall surface portions 153. Spots 100,101, and 102 of the laser beams correspond to the spots of the threelaser beams 34, 35, and 36 shown in FIG. 3, respectively. The spot 101corresponds to the spot of the 0-order diffraction laser beam 35 and theheat treatment or the heat treatment determination is executed by theirradiation of such a laser beam. The spots 100 and 102 correspond tothe spots of the first-order diffraction laser beams 34 and 36 and tracethe land 151 and the groove 152, respectively, so that they are used forthe tracking servo. As will be obviously understood from the diagram, inthis example, an area of the spot 101 on the medium (magnetooptic disk)is smaller than that of each of the spots 100 and 102 on the medium.

FIG. 8B shows an example of a layout of the spots in the case of usingthe heat treatment determining apparatus according to the secondembodiment. According to the construction of the magnetooptic disk shownin the diagram, it is a land/groove substrate similar to that of FIG.11. Spots 110 and 111 of the laser beams correspond to the spots of thelaser beam 87 in FIG. 4. The spot 110 is a spot of the laser beamirradiated from the LD 61 and used for the heat treatment or the heattreatment determination. The spot 111 is a spot of the laser beamirradiated from the LD 80 and used for the tracking servo.

FIG. 9A shows an example of a layout of the spots of the laser beamswhich are irradiated onto the groove substrate or sampling servosubstrate shown in FIG. 5 or 6. A spot 120 is a spot of the laser beamused for the heat treatment. A spot 121 is a spot of the laser beam usedfor the heat treatment determination. In this example, those spots areirradiated from different LDs. It is also possible to separate a laserbeam from one LD into three laser beams by a diffraction grating andallocate two of them as laser beam spots for the heat treatment or theheat treatment determination, respectively. In this example, the spots120 and 121 have almost the same diameter.

FIG. 9B shows an example in which nine laser beams are irradiated onto aland/groove substrate by using two diffraction gratings. The apparatusis constructed in such a manner that the laser beams of spots 130B and132B are used for the tracking, the laser beam of a spot 131B is usedfor the heat treatment, and the laser beam of a spot 131A is used forthe heat treatment determination.

It is also possible to control in such a manner that an area of the spotof the laser beam for the heat treatment on the medium is smaller thanthat of the spot of the laser beam to detect the magnetooptic signal ofthe heat-treated area.

As will be also obvious from the above explanation, many elements amongthe component elements to determine whether the heat treatment is properor improper are common to those used for the heat treatment. Therefore,each of the above embodiments is also constructed so as to realize thedetermining process by the conventional heat treatment apparatus.However, it is unnecessary that the detecting process is limited to sucha construction. For example, the invention can be constructed as amonitoring apparatus for measuring and displaying the level of themagnetooptic signal of the magnetooptic disk or as a dedicated heattreatment determining apparatus for determining whether or not the heattreatment of the magnetooptic disk has properly been executed asnecessary.

1. A heat treatment determining method comprising the steps of:executing a heat treatment of a magnetic layer by irradiating a laserbeam of a first power to an area between tracks of a magnetoopticinformation recording medium obtained by laminating the magnetic layeronto a substrate on which the tracks have previously been formed, inwhich said magnetic layer is constructed by a recording layer to holdrecording magnetic domains according to recording information, adisplacement layer made of a perpendicular magnetic film whose domainwall coercive force is smaller and whose domain wall displacement speedis higher than those of said recording layer, and a switching layerwhich is arranged between said recording layer and said displacementlayer and whose Curie temperature is lower than those of said recordinglayer and said displacement layer; irradiating a laser beam of a secondpower smaller than said first power to said heat-treated area; detectinga level of a magnetooptic signal from reflection light of the laser beamof said second power; and determining whether said heat treatment isproper or improper on the basis of said detected magnetooptic signal. 2.A heat treatment determining method according to claim 1, wherein apredetermined signal is recorded onto said magnetooptic informationrecording medium upon execution, before execution, or after execution ofsaid heat treatment.
 3. A heat treatment determining method according toclaim 1, wherein said magnetooptic information recording medium ismagnetized in one direction upon execution, before execution, or afterexecution of said heat treatment.
 4. A heat treatment determining methodaccording to claim 1, wherein an area of a spot of the laser beam ofsaid second power on said magnetooptic information recording medium islarger than that of a spot of the laser beam of said first power.
 5. Aheat treatment determining apparatus comprising: heat treatment meansfor executing a heat treatment of a magnetic layer by irradiating alaser beam of a first power to an area between tracks of a magnetoopticinformation recording medium obtained by laminating the magnetic layeronto a substrate on which the tracks have previously been formed, inwhich said magnetic layer is constructed by a recording layer to holdrecording magnetic domains according to recording information, adisplacement layer made of a perpendicular magnetic film whose domainwall coercive force is smaller and whose domain wall displacement speedis higher than those of said recording layer, and a switching layerwhich is arranged between said recording layer and said displacementlayer and whose Curie temperature is lower than those of said recordinglayer and said displacement layer; irradiating means for irradiating alaser beam of a second power smaller than said first power to saidheat-treated area; detecting means for detecting a level of amagnetooptic signal from reflection light of the laser beam of saidsecond power; and determining means for determining whether said heattreatment is proper or improper on the basis of said detectedmagnetooptic signal.
 6. A heat treatment determining apparatus accordingto claim 5, wherein a predetermined signal is recorded onto saidmagnetooptic information recording medium upon execution, beforeexecution, or after execution of said heat treatment.
 7. A heattreatment determining apparatus according to claim 5, wherein saidmagnetooptic information recording medium is magnetized in one directionupon execution, before execution, or after execution of said heattreatment.
 8. A heat treatment determining apparatus according to claim5, wherein an area of a spot of the laser beam of said second power onsaid magnetooptic information recording medium is larger than that of aspot of the laser beam of said first power.